docs: def walk(self, into_past=0, into_future=0):
          """
          Walk over a date range and create new instances of self with new ranges.
          """
          walked_range = []
          for shift in range(-into_past, into_future):
              kwargs = dict(drip_model=self.drip_model,
                            name=self.name,
                            now_shift_kwargs={'days': shift})
              walked_range.append(self.__class__(**kwargs))
          return walked_range